Hehe, I've had mine 8 years. Each helmet has an expiration date on the inside tag. Mine is 2001, and has been in numerous minor accidents. I really need a new one. But, I did just buy a show helmet, but that's only for shows.
Yes, it is true. The saying that even if it looks ok on the outside, the inside could easily be damaged. A lady who owns a local tack shop gave a helmet clinic at our fairgrounds a few months back. She had a helmet that was cut in half. The outside looked fine, but once you saw the foam on the inside, it was clearly damaged.
I can't imagine what my helmet looks like inside. I emailed the maker of the helmet (to see if I could get a discount on a new one) and they basically scolded me about my helmet. They said "destroy that helmet and buy a new one."
So, yes. It's time for you and I both to purchase new helmets :)